The present invention relates to a method and a device for determining, from a borehole in a geological environment having at least one heterogeneity, one or more parameters linked to one or more characteristics of the environment or the heterogeneity, such as resistivity, and at least one dimensional parameter, such as the distance between the borehole and the interface of the environment with the heterogeneity.
The invention applies in particular to detecting the stratigraphic beds of an oil reservoir from a borehole. The stratigraphic beds may be a roof, or an intercalated bed, or the interfaces between fluids such as the oil-water interface above or below the borehole.
The evolution in these characteristic dimensional parameters, which are extremely important for hydrocarbon production, allows the following, for example: by tracing the roof and wall, determining the thickness of the reservoir and estimating the reserves there; by tracing an intercalated bed, estimating the hydraulic communication between two parts of the reservoir; by detecting the argillaceous lens, protecting against the cone effect (generally called coning) of water or gas by perforating the pipes and producing the hydrocarbons above or below an argillaceous lens; by tracing the interfaces during production, optimizing production by evolving selective completion; by tracing various geological levels, measurement while drilling and choosing the direction of drilling for optimum positioning.
U.S. Pat. No. 3,838,335 teaches a method and a device for determining the presence of and distance to a heterogeneity from a ratio of electrical resistivities. This method applies in vertical wells, for which the heterogeneity being investigated is an interface perpendicular to the well axis. The method consists of processing the difference in potential between two measuring electrodes on either side of and equidistant from an injection electrode. The evolution in this potential difference as a function of the descent into the well allows the distance separating the well bottom from the heterogeneity investigated to be determined by reading from a system of curves.
Over the last decade, drilling techniques have evolved considerably, particularly the techniques linked to horizontal drilling where it is possible to traverse greater distances across reservoirs, thus obtaining a better picture of the reservoir from the measurements and samples collected in the wells.
At the present time, numerous studies are under way to examine the specific conditions of applications in horizontal wells, and such problems often present difficulties due to the structure and geometry of such wells.
Thus, in the present case where the goal is to obtain a method for determining a parameter linked to one or more characteristics of the environment or the heterogeneity, such as resistivity, and a distance, such as the distance between the borehole and the interface between the environment and the heterogeneity, knowledge of the record specified does not allow an effective method of determination to be obtained. Indeed, when used in a horizontal well, this method is inoperative.
When the heterogeneity investigated is more or less parallel to the well axis, when the emitter is positioned such that it is equidistant between the two receivers, there is no longer any difference in potential between the two measuring electrodes because they are at the same distance from the heterogeneity.
Moreover, the potential difference will be zero whatever the position of the tool in the well.
In addition, the apparatus proposed by U.S. Pat. No. 3,838,335 is obviously based on the use of a cable, which cannot be used in horizontal drilling.